1. Field of the Invention
The present invention relates to the field of materials, in particular glazing panels, which are antisoiling or self cleaning.
2. Description of Related Art
Titanium oxide is known for having photocatalytic properties. In the presence of light, and especially ultraviolet radiation of UV-A type (of which the wavelength is between 320 and 400 nm), titanium oxide has the particular feature of catalyzing the radical degradation reactions of organic compounds. It is known from documents EP-A-850 204 and EP-A-816 466 that titanium oxide also has extremely pronounced hydrophilic properties induced by the same type of radiation. This hydrophilicity, sometimes referred to as “super-hydrophilicity” is characterized by a very small water contact angle, of less than 5°, or even less than 1°. These two properties, photocatalysis on the one hand and super-hydrophilicity on the other hand, give the material containing titanium oxide particularly advantageous properties. Materials, in particular of the ceramic, glass or glass-ceramic type, coated with a thin layer of titanium oxide have, in effect, antisoiling or self cleaning or else ease-of-cleaning properties. A glazing panel covered with a photocatalytic titanium oxide layer degrades the organic soiling which is deposited thereon under the action of sunlight. Mineral soiling is itself partly prevented from being deposited and partly removed owing to the photoinduced super-hydrophilicity of the titanium oxide. Mineral soiling, in certain cases, is in effect caused to be deposited on glazing panels in a form dissolved in raindrops, and precipitates during the evaporation of said drops. Owing to the super-hydrophilicity properties, the water coats and cleans the glazing instead of being deposited thereon in the form of drops, which therefore prevents the deposition of mineral soiling by this mechanism of deposition/evaporation of water drops. As for mineral soiling such as dust which is deposited without the aid of rain, for example under the action of wind, it is quite simply removed by rainwater runoff. The materials obtained therefore allow the removal of organic and mineral soiling under the combined effect of solar radiation and water, in particular rain, runoff.
The materials described previously have, however, a drawback when they are placed in a site protected from the rain or in a geographical zone that very rarely experiences precipitation. Specifically, it has been observed that, placed in an atmosphere rich in mineral soiling and sheltered from the rain, glazing panels covered with photocatalytic titanium oxide and that are super-hydrophilic are gradually covered with mineral soiling, in particular in the form of very adherent dust. After long-term exposure, especially for more than 2 months, or even 4 months, such glazing has a surface that is as dirty as that of a glazing panel devoid of a titanium oxide layer. In the absence of water runoff, the super-hydrophilic titanium oxide surface does not therefore prevent the deposition and adhesion of mineral soiling.